Remove unused LocalSurfaceId

Whle reading through the code, noticed this was entirely unused.

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel
Change-Id: Iacb86674e3003ee12b44b6261b1dfa01086f7ba2
Reviewed-on: https://chromium-review.googlesource.com/934961Reviewed-by: Chris Blume <cblume@chromium.org>
Commit-Queue: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#538911}

cc/trees/layer_tree_host_impl.h

@@ -811,8 +811,6 @@ class CC_EXPORT LayerTreeHostImpl
 
   LayerTreeFrameSink* layer_tree_frame_sink_;
 
-  viz::LocalSurfaceId local_surface_id_;
-
   // The following scoped variables must not outlive the
   // |layer_tree_frame_sink_|.
   // These should be transfered to viz::ContextCacheController's